In prior art video signal separating systems, it is known to utilize horizontal correlation, vertical correlation and time (frame) correlation of both the luminance signal Y and a carrier chrominance signal (chroma signal) C to separate them. Correlation is meant how the signals change along a line, from line to line, or from frame to frame, respectively. Known Y/C separating circuits utilizing the above respective correlations will hereinafter be described with reference to FIGS. 8 to 10.
FIG. 8A illustrates a horizontal correlation type Y/C separating circuit. Referring to FIG. 8A, a video signal applied to an input terminal 1 is supplied in common to a low pass filter 2Y and a bandpass filter 2C. From an output of the low pass filter 2Y, a luminance signal Y.sub.H is supplied to an output terminal 3Y, whereas from an output of the bandpass filter 2C, a chroma signal C.sub.H is supplied to an output terminal 3C.
FIG. 8B illustrates a vertical correlation type Y/C separating circuit. Referring to FIG. 8B, a video signal applied to an input terminal 1 is supplied in common to a 1H delay line 4, an adder 5Y and a subtracter 5C. The output from the 1H delay line 4 is supplied in common to the adder 5Y and the subtracter 5C. As a result, a luminance signal Y.sub.V from the adder 5Y is supplied to an output terminal 6Y and a chroma signal C.sub.V from the subtracter 5C is supplied to an output terminal 6C.
FIG. 8C illustrates a frame correlation type Y/C separating circuit. Referring to FIG. 8C, a video signal applied to an input terminal 1 is supplied in common to an adder 8Y, a subtracter 8C and a frame memory 7 which constitutes a one frame delay line. The output of the frame memory 7 is supplied to the adder 8Y and the subtracter 8C. As a result, a luminance signal Y.sub.F from the adder 8Y is supplied to an output terminal 9Y and a chroma signal C.sub.F from the subtracter 8C is supplied to an output terminal 9C.
While the frame memory 7 is inherently designed to process a digitized signal, it is arranged in the circuit of the invention so as to process an analog signal for the sake of simplicity. Therefore, an A/D (analog-to-digital) converter at the input side thereof and a D/A (digital-to-analog) converter at the output side thereof are not shown but are understood to be included.
FIGS. 9A to 9C illustrate frequency vs. amplitude characteristics of the Y/C separating circuits shown in FIGS. 8A to 8C, respectively.
In FIG. 9A, a bold solid curve indicates the pass band characteristic of the low pass filter 2Y and a dashed curve the pass band characteristic of the bandpass filter 2C. In FIG. 8A, the low pass filter 2Y can be replaced by a subcarrier trap. In this case, the pass band characteristic is indicated as both bold and fine solid curves in FIG. 9A.
The vertical correlation type Y/C separating circuit of FIG. 8B constitutes a known comb filter. The pass band characteristic of this comb filter relative to the luminance signal Y.sub.V is made, as shown by a solid curve in FIG. 9B, to have the maximum attenuation degree at a subcarrier frequency fsc and respective points spaced apart therefrom by the distance of some integer times the horizontal frequency f.sub.H and the minimum attenuation degree at each of intermediate points between these points, i.e. at each of the points spaced apart from the subcarrier frequency fsc by the distance of an odd number times 1/2 of the horizontal frequency f.sub.H : (2n+1) f.sub.H /2. On the other hand, the pass band characteristic thereof relative to the chroma signal C.sub.V is made, as shown by a dashed curve in FIG. 9B, to have a minimum attenuation degree at the subcarrier frequency fsc and respective points spaced apart therefrom by the distance of some integer times the horizontal frequency f.sub.H and a maximum attenuation degree at each of the points spaced apart from the subcarrier frequency fsc by the distance of an odd number times the horizontal frequency f.sub.H, i.e. (2n+1) f.sub.H /2.
The reason for this is that by the known technique of frequency interleaving, as shown by dashed line in FIG. 10A, the spectrums of the chroma signal C are located between the spectrums of the luminance signal Y shown by a solid line in the same figure.
The frame correlation type Y/C separating circuit of FIG. 8C forms a known comb filter. The pass band characteristic of this comb filter relative to the luminance signal Y.sub.F is made, as shown by a solid curve in FIG. 9C, to have the maximum attenuation degree at the subcarrier frequency fsc and respective points spaced apart by the distance of some integer times the frame frequency f.sub.F (=f.sub.V /2) and the minimum attenuation degree at each of the intermediate points between these points, that is, points spaced apart from the subcarrier frequency fsc by the distance of an odd number times 1/2 of the frame frequency f.sub.F, i.e. (2n+1) f.sub.F /2. On the other hand, the pass band characteristic thereof relative to the chroma signal C.sub.F is made, as shown by a dashed line in FIG. 9C, to have the minimum attenuation degree at the subcarrier frequency fsc and respective points spaced apart therefrom by the distance of some integer times the frame frequency f.sub.F and the maximum attenuation degree at respective points spaced apart from the subcarrier frequency fsc by the distance of odd times of 1/2 of the frame frequency f.sub.F, i.e. (2n+1) f.sub.F /2.
The reason for this will be understood as follows. As shown in FIG. 10B, which is a partially enlarged view of FIG. 10A, by the known technique of frequency interleaving, a side band wave (shown by an open circle) of the vertical frequency f.sub.V accompanied with an odd higher harmonic wave of the horizontal frequency f.sub.H and a side band wave (shown by a solid circle) of the vertical frequency f.sub.V accompanied with an even higher harmonic wave are spaced apart from each other by a distance of the frame frequency f.sub.F =f.sub.V /2. Then, the subcarrier frequency fsc and the side band wave of the vertical frequency f.sub.V accompanied therewith are located between the frequency intervals of the frame frequency f.sub.F.
However, these known Y/C separating apparatus, individually utilizing various correlations, cause the quality of the reproduced picture to be deteriorated at the places when no correlation exists.
When horizontal correlation is utilized, the deterioration of the quality of picture, such as the deterioration of a frequency characteristic and the occurrence of crosstalk, occurs on the picture at its right and left side edges of the longitudinal stripes as shown in FIG. 11A when the levels of the luminance signal Y and the chroma signal C are changed abruptly in the horizontal direction.
When vertical correlation is utilized, the deterioration of the quality of picture, such as the occurrence of crosstalk and the appearance of dots of the subcarrier, occurs on the picture at the top and bottom edges of the horizontal stripes as shown in FIG. 11B when the levels of the luminance signal Y and the chroma signal C are abruptly changed in the vertical direction.
When frame correlation is utilized, as shown in FIGS. 11C1 to 11C3, crosstalk occurs in part or wholly in the peripheral edge of the picture in relation to the direction in which a figure in the picture is moved in the directions shown by arrows and also the quality of picture is considerably deteriorated, such as multiple-line interference or the like when the peripheral edge of the figure is doubled or tripled.
In order to solve such problems, it may be proposed to switch the separated outputs in response to the moving amount of the picture element at a proper timing. In this case, the signal processing apparatus needs a moving picture element detecting section which would cause the arrangement of the apparatus to be large in size and complicated.